code: def test_abs():
      z = abs(datum.xxx)
      assert repr(z) == "abs(datum.xxx)"
  
file_path: altair/tests/expr/test_expr.py
imports: import operator
  import pytest
  from altair import expr
  from altair import datum
  from altair import ExprRef
  from jsonschema.exceptions import ValidationError
label_desc: Use image node_id 3 for calling a global function with example usage: test_abs() without return types